In preparing a sewing machine for stitching material, it is necessary to carry out a series of threading procedures including threading a needle thread withdrawn from a spool along a predetermined threadline in a predetermined sequence via threading members including tension disks, a thread take-up spring and a thread take-up lever, and threading the needle thread through the eye of the needle.
In a conventional sewing machine, the needle thread released from the loop seizing beak of the shuttle is sometimes caught by a part of the shuttle when the needle thread is suddently pulled upwardly by the take-up lever in order to tighten the needle thread, and therefore, in some cases, an excessive length of the needle thread is withdrawn from the spool. In such a case, the stitch is not tightened sufficiently, and hence proper stitches are not formed.
Generally, unskilled operators are liable to achieve improper or correct threading results, and the threading operation is one of the most troublesome tasks even to skilled operators. It is of course essential to carry out the threading operation correctly, because a proper sewing operation is impossible to achieve when the threading operation is defective.
When threading the take-up lever, for example, the take-up lever must be disposed at its uppermost position. If the take-up lever is threaded when disposed at a position other than its uppermost position, the take-up lever moves upwardly toward the uppermost position at the start of the sewing operation thereby tending to withdraw the thread from the eye of the needle. Accordingly, an ordinary sewing machine has a mechanism which is specifically designed to bring the take-up lever to its uppermost position when the sewing machine is stopped.
However, since it is difficult to incorporate such a mechanism into a relatively expensive sewing machine, the operator is required to perform such a troublesome operation every time a threading operation is necessary. In particular, the operator has to manually rotate the hand pulley so as to bring the take-up lever to its uppermost position. Even with a sewing machine having a mechanism to bring the take-up lever to automatically the uppermost position when the sewing machine is stopped, the same manual operation is necessary to rotate the hand pulley, if prior to threading, the hand pulley is accidentally turned, thereby moving the take-up lever downwardly from the uppermost position.